It is well known in the art to heat seal plastic sheets together, and especially tough, transparent plastic film sheets of polyester films. These polyester films are generally polyethylene terephthalate films, and a typical example is the polyester film known in the trade as "Mylar," a product of E. I. duPont de Nemours & Company. Note, for instance, U.S. Pat. No. 3,866,648, directed to a microfilm copy jacket made of polyester films which are held together by sealing lines formed by heat or sonic sealing to fuse the two films together at the point of contact.
One type of commercial polyester heat sealing machine utilizes a nickel-chrome heating ribbon which is maintained in a taut condition under tension. A polyester film can be abutted against the ribbon for alignment purposes, and then a clamp is lowered to clamp the polyester sheet to a work table surface for the heat sealing step. In this regard, it has generally been recognized by those in the art that poor quality seals will be obtained unless some type of clamping is utilized, when two or more polyester sheets are to be heat sealed together. As the polyester sheets are being clamped to the work table surface, the nickel-chrome ribbon heating element is moved away from the clamped polyester sheet edges which had contacted the ribbon during the alignment step mentioned above. Typically, the ribbon would be moved away from the polyester film sheet edge a distance of 1/16 to 3/16 of an inch. Then the heating cycle is initiated, and the heating ribbon heats the polyester film sheet edges to a temperature such that the films are fused together along the edge exposed to the heat sealing step. When the heat cycle is complete, the heat sealed sheets are permitted to cool somewhat in the heat sealing machine, and then the clamping pressure is released.
In such a device, when the clamping pressure is released the still-cooling nickel-chrome ribbon is moved into abutting contact with the freshly-sealed sheet edge, and if the film or the ribbon has not cooled sufficiently, the film sheet edge may tend to adhere to the ribbon, resulting in an unacceptable product. Furthermore, to aid the cooling step after the heat sealing cycle, a fan is generally in continuous operation except during the heating step, blowing room air across the location where the polyester film sheet edge is located during the heating and cooling cycles. Because of the air flow caused by the fan, however, the next, unsealed polyester film sheets may be difficult to align, with the alignment being maintained until the sheets are firmly clamped to proper position. Furthermore, with such an arrangement only one side of the nickel-chrome heating ribbon can be utilized in a given heat sealing cycle, so that the resulting production rates from such a heat sealing machine are low.